Stepping motor type indicator having a pole tooth with reduced end area

ABSTRACT

A stepping motor type indicator provided with unambiguous stable points having a holding force in a non-excited condition sufficient to suppress undesirable movements of a pointer. The stepping motor type indicator includes a stepping motor ( 2 ) having a rotor ( 28 ) with an output shaft ( 27 ) and stators (A, B). Yokes ( 23, 26 ) have a plurality of pole teeth ( 232, 233, 262, 263 ) extending toward curved outer surfaces of the rotor. The pole ends of a specific pole tooth ( 232   a ) chosen from one of the plurality of pole teeth is made to have a pole end area different than that of the remaining pole teeth. This structure produces unambiguous stable points (“a”-“l”) and produces strong holding forces so that an attractive force exerted by the stable point (“a”) on the pointer ( 4 ) toward stopper pin ( 7 ) is increased to positively suppress undesirable movements of pointer ( 4 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stepping motor type indicators forindicating various types of varying measurements, such as the runningspeed or engine speed of a vehicle, by the pointing direction or angleof a pointer which swings over a dial. More particularly, the inventionpertains to a structure for supporting the pointer.

2. Description of the Related Art

A PM type stepping motor is a device widely used as an actuator inconventional office automation equipment, home electronic appliances andmotor vehicles, for instance. The PM-type stepping motor allows easycontrol of forward and reverse running operation and its physical sizecan be easily reduced depending on required driving torque. For thisreason, the PM-type stepping motor attracts growing attention in recentyears as a prime mover for instruments as a substitute for air-core typemovements of cross coil instruments.

As disclosed in JP-A-6-153484, for instance, a stepping motor of thistype mostly comprises a rotor having an output shaft and constructedessentially of a permanent magnet and a stator formed of a yoke havingan array of pole teeth arranged face to face with a curved outer surfaceof the rotor and exciting coils for magnetizing the pole teeth, whereindriving signals having controlled phase differences are fed into theindividual exciting coils of the stator to magnetize the pole teeth andthereby produce a rotating magnetic field, which causes the rotor torotate.

With a pointer fixed to the output shaft and a dial plate fixed betweenthe pointer and the stepping motor, the driving signals fed into theexciting coils are controlled in accordance with a measurement value(e.g., the varying running speed or engine speed of a vehicle), wherebythe motor can be used as a prime mover of an indicator.

In a case where the stepping motor is used as a prime mover of anindicator as described above, it is necessary to hold the pointerstationary at a specified position over the dial plate and suppressundesirable movements of the pointer during a period when no drivingvoltage signals are applied to the exciting coils. An effective meansfor achieving this is rotor (pointer) arresting means which mechanicallystops pointer movements.

In the stepping motor, however, complicated magnetic circuits are formedbetween the rotor and the stator including the pole tooth array and themagnetic circuits produce magnetic forces which act on the rotor evenunder a non-excited condition, although the magnetic forces areconsiderably small. Therefore, with the provision of the rotor arrestingmeans alone, the pointer is apt to depart from the position where it wasset by the rotor arresting means. In this circumstance, there has beenthe need for an effective means of positively holding the pointer at aspecified position.

Taking into consideration the aforementioned problem of the prior art,the applicants of the present invention previously proposed inJP-A-9-184740 an arrangement for holding a pointer at a specifiedposition by using magnetic forces exerted on a rotor under thenon-excited condition. This previous arrangement was devised in thelight of the fact that the magnetic forces exerted by a yoke on therotor under the non-excited condition produce a plurality of stablepoints which attract and hold the pointer (rotor). More specifically, astopping position of the pointer determined by rotor arresting means isset at a point located on a decreasing side of a middle point betweentwo adjacent stable points existing along increasing and decreasingdirections of the pointer in the previous arrangement.

However, the strength of holding forces of such stable points coulddiffer from one product to another due to assembly errors or dimensionalerrors of constituent components, or the stable points which could forceor attract the rotor to specific positions might not be positivelyformed in certain cases. Even when a stopping position of the pointer isset in consideration of the location of the stable points as describedabove, it might not be possible to positively hold the pointer at theset stopping position if the stable points formed are ambiguous and theholding forces of the stable points are too small. It is understood fromthe above discussion that there are limitations in the aforementionedprevious arrangement as the means of holding the pointer at a specifiedposition under non-excited conditions and, therefore, there has been acontinuing need for improvement.

It is an object of the invention to provide a stepping motor typeindicator whose stepping motor forms unambiguous stable points whichproduce a sufficiently strong holding force in a non-excited condition,making it possible to suppress undesirable movements of a pointer.

BRIEF SUMMARY OF THE INVENTION

To achieve this object, a stepping motor type indicator in one aspect ofthe invention comprises a stepping motor which is constructed of a rotorformed of permanent magnets and having an output shaft, and a statorhaving a yoke, from which a plurality of pole teeth extend toward acurved outer surface of the rotor, and an exciting coil for magnetizingthe yoke, in which the rotor is driven to rotate in accordance with ameasurement value, a pointer which is connected to the output shaft ofsaid stepping motor and indicates the measurement value on a dial plate,and a rotor stopper for stopping said pointer at a specified stoppingposition on the dial plate, wherein the stopping position of saidpointer determined by said rotor stopper is set at a point closer to astable point which is located on a decreasing side of dial reading ofsaid pointer than an approximately middle point between two adjacentstable points arbitrarily selected from a plurality of stable points ofthe rotor which are formed along the turning direction of said pointerdue to magnetic forces exerted between the rotor and the yoke in anon-excited condition of the exciting coil, and the pole end area of aspecific pole tooth chosen from said multiple pole teeth facing therotor is made different from the pole end areas of the other pole teethfacing the rotor.

In another aspect of the invention, a stepping motor type indicatorcomprises a stepping motor which is constructed of a rotor formed ofpermanent magnets and having an output shaft, and first and secondstators individually having yokes, from which a plurality of pole teethextend toward a curved outer surface of the rotor, and exciting coilsfor magnetizing the yokes, in which said first and second stators areconcentrically stacked and said rotor is driven to rotate in accordancewith a measurement value, a driver which supplies driving signals to theexciting coils of said individual stators to cause the rotor to rotatein accordance with the measurement value, a pointer which is connectedto the output shaft of said stepping motor and indicates the measurementvalue on a dial plate, and a rotor stopper for stopping said pointer ata specified stopping position on the dial plate, wherein the stoppingposition of said pointer determined by said rotor stopper is set at apoint closer to a stable point which is located on a decreasing side ofdial reading of said pointer than an approximately middle point betweentwo adjacent stable points arbitrarily selected from a plurality ofstable points of the rotor which are formed along the turning directionof said pointer due to magnetic forces exerted between the rotor and theyokes in a non-excited condition of the exciting coils, the pole endarea of a specific pole tooth chosen from said multiple pole teethfacing the rotor is made different from the pole end areas of the otherpole teeth facing the rotor so that a relative difference is createdbetween the pole end areas of the pole teeth of said first stator facingthe rotor and those of the pole teeth of said second stator, and theroot-mean-square value of the driving signal fed into the exciting coilof said first stator whose pole teeth have smaller pole end areas facingthe rotor is made higher than the root-mean-square value of the drivingsignal fed into the exciting coil of said second stator whose pole teethhave larger pole end areas facing the rotor.

In one specific form of the invention, the pole end area of only onespecific pole tooth is made different from the pole end areas of theother pole teeth.

In a further aspect of the invention, a stepping motor type indicatorcomprises a stepping motor which is constructed of a rotor formed ofpermanent magnets and having an output shaft, and first and secondstators individually having yokes, from which a plurality of pole teethextend toward a curved outer surface of the rotor, and exciting coilsfor magnetizing the yokes, in which said first and second stators areconcentrically stacked and said rotor is driven to rotate in accordancewith a measurement value, a driver which supplies driving signals whoseoutput waveforms have a phase difference of about 90< in terms ofelectrical angle to the exciting coils of said individual stators tocause the rotor to rotate in accordance with the measurement value, apointer which is connected to the output shaft of said stepping motorand indicates the measurement value on a dial plate, and a rotor stopperfor stopping said pointer at a specified stopping position on the dialplate, wherein the stopping position of said pointer determined by saidrotor stopper is set at a point closer to a stable point which islocated on a decreasing side of dial reading of said pointer than anapproximately middle point between two adjacent stable pointsarbitrarily selected from a plurality of stable points of the rotorwhich are formed along the turning direction of said pointer due tomagnetic forces exerted between the rotor and the yokes in a non-excitedcondition of the exciting coils, the pole end area of a specific poletooth chosen from said multiple pole teeth facing the rotor is madedifferent from the pole end areas of the other pole teeth facing therotor, and an angular position taken by the said pointer when saiddriver outputs the driving signals corresponding to an electrical angleof 0° and said stable point in the non-excited condition are matchedexactly or approximately with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a stepping motor typeindicator according to a preferred embodiment of the invention;

FIG. 2 is an exploded perspective view of a stepping motor shown in FIG.1;

FIG. 3 is a cross-sectional view showing how the stepping motor of FIG.2 is assembled;

FIG. 4 is a cross-sectional view showing how the stepping motor typeindicator of FIG. 1 is assembled;

FIG. 5 is a block diagram showing a driving system of the stepping motortype indicator;

FIG. 6 is a waveform diagram showing output waveforms of driving means;

FIG. 7(a) to FIG. 7(c) are elevational views showing the operation ofthe stepping motor type indicator of the embodiment;

FIG. 8 is a partially cutaway elevational view showing a relationshipbetween a stopping position of a pointer (rotor) determined by rotorarresting means and stable points;

FIG. 9 is a development showing the arrangement of individual poleteeth; and

FIG. 10(a) is a fragmentary elevational view showing a relationshipbetween a stable point and an angular position taken by the pointer whendriving signals having a phase difference of 0° in terms of electricalangle is entered, and FIG. 10(b) is a fragmentary elevational viewshowing a comparative example of FIG. 10(a).

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, a stepping motor type indicator comprises astepping motor 2 which is constructed of a rotor 28 having an outputshaft 27 and first and second stators A, B having yokes 23, 26, fromwhich a plurality of pole teeth 232, 233, 262, 263 extend toward acurved outer surface of the rotor 28, and exciting coils 21, 24 formagnetizing the yokes 23, 26, respectively, wherein the rotor 28 isdriven to rotate in accordance with a measurement value, a pointer 4which is connected to the output shaft 27 of the stepping motor 2 andindicates the measurement value on a dial plate 3, and a stopper pin(rotor arresting means) 7 for stopping the pointer 4 at a specifiedstopping position P on the dial plate 3. When the exciting coils 21, 24are not excited, there are formed a plurality of stable points a-l ofthe rotor 28 along the turning direction of the pointer 4 due tomagnetic forces exerted between the rotor 28 and the yokes 23, 26. Thestopping position P of the rotor 28 (pointer 4) determined by thestopper pin 7 is set at a point closer to the stable point a which islocated on a decreasing side of dial reading than an approximatelymiddle point M between arbitrarily selected two adjacent stable points(a and b in this example). Since the pole end area of a specific poletooth 232 a chosen from the multiple pole teeth 232, 233, 262, 263facing the rotor 28 is made different from the pole end areas of theother pole teeth 232, 233, 262, 263 facing the rotor 28 in thisconstruction, the individual stable points a-l become unambiguous andproduce strong holding forces so that an attracting force exerted by thestable point a on the pointer 4 (rotor 28) toward the stopper pin 7 isincreased, making it possible to positively suppress undesirablemovements of the pointer 4.

In a case where the individual pole teeth 232, 233, 262, 263 have thesame pole end area, it is unlikely that large differences occur indetent torques produced by magnetic circuits formed between individualmagnetic poles of the rotor 28 and their corresponding pole teeth 232,233, 262, 263 and, therefore, the detent torques are generally balancedaround the curved outer surface of the rotor 28. In the aforementionedconstruction, however, there is made a distinct difference between thedetent torque produced by the pole tooth 232 a whose pole end areadiffers and the detent torques produced by the other pole teeth 232,233, 262, 263. Such variations in the detent torque create highlydistinguishable relative differences between unstable points M (whichexert smaller attracting forces on the rotor 28) and the stable pointsa-l (which exert larger attracting forces on the rotor 28) which areformed at specific intervals along the rotating direction of the rotor28. For this reason, the stable points a-l become more distinct and, asa consequence, the attracting force exerted on the pointer 4 (rotor 28)by the stable point a toward the stopper pin 7 is increased andundesirable movements of the pointer 4 can be positively suppressed.

As the pole end area of the specific pole tooth 232 a chosen from themultiple pole teeth 232, 233, 262, 263 facing the rotor 28 is madedifferent from the pole end areas of the other pole teeth 232, 233, 262,263 as described above, there is created a relative difference betweenthe pole end areas of the pole teeth 232 (232 a), 233 of the firststator A facing the rotor 28 and those of the pole teeth 262, 263 of thesecond stator B. Since the root-mean-square value of a driving signalfed from driving means 800 into the exciting coil 21 of the first statorA whose pole teeth 232 (232 a), 233 have smaller pole end areas facingthe rotor 28 is made higher than the root-mean-square value of a drivingsignal fed into the exciting coil 24 of the second stator B whose poleteeth 262, 263 have larger pole end areas facing the rotor 28, itbecomes possible to maintain good excitation balance between theindividual stators A, B in a normal driven condition (excited condition)and thereby suppress such adverse effects on rotary motion of the rotor28 as an increase in the indication error of the pointer 4 anddegradation of smoothness of the movement of the pointer 4.

When the pole end area of the specific pole tooth 232 a facing the rotor28 is made different from the pole end areas of the other pole teeth232, 233, 262, 263 and there is a relative difference between the poleend areas of the pole teeth 232 (232 a), 233 of the first stator Afacing the rotor 28 and those of the pole teeth 262, 263 of the secondstator B, a difference in excitation torque could occur between the twostators A, B in a normal operating condition (excited condition),affecting rotational characteristics of the rotor 28 (pointer 4) incertain cases. In the aforementioned construction, because theroot-mean-square value of the driving signal fed into the exciting coil21 of the first stator A whose pole teeth 232 (232 a), 233 have smallerpole end areas facing the rotor 28 is made higher than theroot-mean-square value of the driving signal fed into the exciting coil24 of the second stator B whose pole teeth 262, 263 have larger pole endareas facing the rotor 28, the excitation torque produced by the firststator A is increased relative to the excitation torque produced by thesecond stator B to thereby achieve a proper balance of excitation torquebetween the two stators A, B. This makes it possible to suppress suchadverse effects on the rotational characteristics of the rotor 28 as anincrease in the indication error of the pointer 4 and degradation ofsmoothness of the movement of the pointer 4.

In addition, because the pole end area of only one specific pole tooth232 a is made different from the pole end areas of the other pole teeth232, 233, 262, 263, adverse effects on the rotational characteristics ofthe rotor 28 in the normal operating condition (excited condition) canbe reduced.

More specifically, although the rotational characteristics of the rotor28 (pointer 4) in the normal operating condition (excited condition)would be affected if the pole end areas of some of the pole teeth 232,233, 262, 263 are differentiated from the pole end areas of the otherpole teeth 232, 233, 262, 263, such adverse effects on the rotationalcharacteristics of the rotor 28 during operation in the excitedcondition as an increase in the indication error of the pointer 4 anddegradation of smoothness of the movement of the pointer 4 can beminimized when the pole end area of only one pole tooth 232 a is madedifferent from the pole end areas of the other pole teeth 232, 233, 262,263.

In a case where the driving means 800 for driving the stepping motor 2supplies the driving signals whose output waveforms have a phasedifference of about 90° in terms of electrical angle, it is possible toprevent the pointer 4 from moving from its stopping position Pdetermined by the stopper pin 7 when the driving means 800 outputs thedriving signals corresponding to an electrical angle of 0° (when anignition switch 84 is turned on, for example), if an angular positiontaken by the pointer 4 when the driving means 800 outputs the drivingsignals corresponding to the electrical angle of 0° and the stable pointa produced in the non-excited condition are matched exactly orapproximately with each other.

A stepping motor type indicator according to a preferred embodiment ofthe invention is now described referring to the accompanying drawings.

FIG. 1 is an exploded perspective view of a stepping motor typeindicator according to the preferred embodiment of the invention; FIG. 2is an exploded perspective view of a stepping motor 2 shown in FIG. 1;FIG. 3 is a cross-sectional view showing how the stepping motor 2 ofFIG. 2 is assembled; FIG. 4 is a cross-sectional view showing how thestepping motor type indicator of FIG. 1 is assembled; FIG. 5 is a blockdiagram showing a driving system of the stepping motor type indicator;FIG. 6 is a waveform diagram showing output waveforms of driving means;FIGS. 7(a)-7(c) are elevational views showing the operation of thestepping motor type indicator of the embodiment; FIG. 8 is a partiallycutaway elevational view showing a positional relationship between thestepping motor 2 (rotor 28) and a dial plate 3 (rotor arresting means);and FIG. 9 is a development showing the arrangement of individual poleteeth.

In FIG. 1, the stepping motor type indicator constitutes a speedometerfor indicating the running speed of a motor vehicle, for example. Thestepping motor type indicator comprises the stepping motor 2 mounted ona support member 1, which is formed of a printed wiring board, forinstance, the aforementioned dial plate 3 positioned in front of thestepping motor 2, the dial plate 3 bearing on its outer surface a scale30 formed of graduation marks, numerals and other markings, and apointer 4 which is driven by the stepping motor 2.

Referring to FIG. 2, the stepping motor 2 comprises a first stator Aincluding a first exciting coil 21 wound around a first ring-shapedbobbin 22 made of a plastic material which is fitted in a firstring-shaped yoke 23 made of an iron-based metallic material, such asSEC, SPC or permalloy, a second stator B including a second excitingcoil 24 wound around a second ring-shaped bobbin 25 made of a plasticmaterial which is fitted in a second ring-shaped yoke 26 made of aniron-based metallic material, such as SEC, SPC or permalloy, a rotor 28formed of permanent magnets having a plurality of magnetic poles whichare arranged at regular intervals, an output shaft 27 being fixed on ancentral axis of the second stator B, and a pair of supporting plates 29for supporting the output shaft 27 of the rotor 28.

In the first stator A, the first ring-shaped bobbin 22 is formed of acylindrical winding section 220 and a pair of disk-like winding guides221, 222 provided at both ends of the winding section 220.

The first ring-shaped yoke 23 is constructed of a generally cup-shapedfirst outer yoke element 230 and a generally sheet-like first inner yokeelement 231 whose outside diameter is slightly larger than the insidediameter of an opening of the first outer yoke element 230. The firstring-shaped bobbin 22 is accommodated in the first outer yoke element230 and held between the first outer yoke element 230 and the firstinner yoke element 231.

In central parts of the first outer and inner yoke elements 230, 231,there are formed a plurality of pole teeth 232, 233 extending inwardalong a curved inner surface of the winding section 220 of the firstring-shaped bobbin 22. These pole teeth 232, 233 are formed into thegenerally same shape and arranged at generally equal intervals. The poleteeth 232, 233 of the first outer and inner yoke elements 230, 231 aremeshed with one another like alternate comb teeth, together forming anannular array whose diameter is slightly larger than the outsidediameter of the rotor 28.

In the second stator B, the second ring-shaped bobbin 25 is formed of acylindrical winding section 250 and a pair of disk-like winding guides251, 252 provided at both ends of the winding section 250.

Like the first ring-shaped yoke 23, the second ring-shaped yoke 26 isconstructed of a generally cup-shaped second outer yoke element 260 anda generally sheet-like second inner yoke element 261 whose outsidediameter is slightly larger than the inside diameter of an opening ofthe second outer yoke element 260. The second ring-shaped bobbin 25 isaccommodated in the second outer yoke element 260 and held between thesecond outer yoke element 260 and the second inner yoke element 261.

In central parts of the second outer and inner yoke elements 260, 261,there are formed a plurality of pole teeth 262, 263 extending inwardalong a curved inner surface of the winding section 250 of the secondring-shaped bobbin 25. These pole teeth 262, 263 are formed into thegenerally same shape and arranged at generally equal intervals. The poleteeth 262, 263 of the second outer and inner yoke elements 260, 261 aremeshed with one another like alternate comb teeth, together forming anannular array whose diameter is slightly larger than the outsidediameter of the rotor 28.

The first stator A constructed of the first ring-shaped bobbin 22 andring-shaped yoke 23 and the second stator B constructed of the secondring-shaped bobbin 25 and ring-shaped yoke 26 are concentrically stackedas shown in FIG. 3, wherein the openings of the individual outer yokeelements 230, 260 are connected by electric welding, forming a hollowspace S inside, in which the rotor 28 is rotatably accommodated withboth ends of the output shaft 27 supported by the supporting plates 29.The number of the pole teeth 232, 233 and of the pole teeth 262, 263 ismade equal to the number of the magnetic poles of the rotor 28 and thepole teeth 262, 263 are offset from the pole teeth 232, 233 by as muchas one-quarter of their tooth interval in the rotating direction ofrotor 28. Further, the pole end areas of the pole teeth 232, 233, 262,263 are made approximately equal to each other except the pole end areaof the specified pole tooth 232 a as will be described later.

Referring to FIGS. 1 and 4, the support member 1 and the dial plate 3are fixed to the individual supporting plates 29 of the stepping motor 2by tapping screws 5 and 6, respectively. On the other hand, the pointer4 is firmly press-fitted to a far end of the output shaft 27 of thestepping motor 2 that extends passing through the dial plate 3. Astopper pin (rotor arresting means) 7 extending in the axial directionof the output shaft 27 is fixed on the outer surface of the dial plate3. As will be described later, the stopper pin 7 serves to stop thepointer 4 at a specified position when the stepping motor type indicatoris powered off as the pointer 4 comes into contact with the stopper pin7 at power-off (non-excited condition). It is possible to employ adirect rotor stopper which comes into direct contact with rotor 28 tostop its motion or a direct shaft stopper (not shown) which comes intodirect contact with a dedicated moving contact piece (not shown)attached to the output shaft 27 to stop its motion as rotor arrestingmeans instead of the stopper pin.

The stepping motor type indicator thus constructed is driven by adriving system including the later-described driving means, in whichdriving signals according to a measurement value are supplied to theexciting coils 21, 24 and their magnetic paths are lead to the magneticpoles of the rotor 28 through the pole teeth 232, 233, 262, 263 of theindividual ring-shaped yokes 23, 26, causing the rotor 28 to rotate ineither direction as appropriate and the pointer 4 linked to the rotor 28via the output shaft 27 to swing in accordingly. The pointer 4 whichswings on the scale 30 marked on the dial plate 3 in this mannerindicates the measurement value.

Operation of the stepping motor type indicator as it is used as aspeedometer of a motor vehicle is described more specifically referringto FIG. 5. When a frequency signal which is proportional to themeasurement value, or the running speed of the vehicle, is entered froman input terminal 80, a counter block 81 detects leading and trailingedges of the input signal, counts them throughout a specified gate time(gate time method) or counts a separate high-frequency clock signal bythe input signal (period measurement method), and calculates thecontinuously varying running speed in the form of digital data.

The digital data which is the measurement value thus obtained isconverted into indication angle data at specified conversion timeintervals by a processing block 82, and the absolute value of thedifference between the latest indication angle data and precedingindication angle data and increment/decrement status of the indicationangle data are calculated at the conversion time intervals.

The latest indication angle data is used as the preceding indicationangle data when the indication angle data is updated next. The latestand preceding indication angle data are repeatedly compared in thismanner in each successive updating cycle.

The indication angle data thus updated is fed into an output block 83which performs specific waveform processing (e.g., microstep operationand phase-converting operation) and supplied to the exciting coils 21,24 of the stepping motor 2 as the driving signals (voltage signals). Asa consequence, the exciting coils 21, 24 are excited and the magneticpaths are lead to the rotor 28 through the pole teeth 232, 233, 262, 263of the individual ring-shaped yokes 23, 26 to drive the rotor 28,whereby the pointer 4 fixed to the output shaft 27 of the rotor 28swings to an angular position corresponding to a correct speed readouton the scale 30 on the dial plate 3. In this embodiment, the counterblock 81, the processing block 82 and the output block 83 together formdriving means 800 for driving the stepping motor type indicator. In FIG.5, the numeral 84 designates an ignition switch serving as a powerswitch and the numeral 85 designates an onboard battery serving as apower source.

The driving signals processed by the output block 83 are voltage signalsshaped generally into sine and cosine waves having a phase difference of90° in terms of electrical angle as shown in FIG. 6. These voltagesignals are actually microstep waves which rise and fall in discretesmall steps along the sine and cosine waves. As will be described later,the root-mean-square value of the driving signal fed into the firstexciting coil 21 of the first stator A is made higher than theroot-mean-square value of the driving signal fed into the exciting coil24 of the second stator B in this embodiment. For example, theroot-mean-square values of the driving signals fed into the first statorA and the second stator B are set to 7 V and 6.5 V, respectively.

The processing block 82 together with the counter block 81 may be formedof a microcomputer so that desired indicating characteristics can beprogrammed to allow the stepping motor type indicator to properlyindicate the running speed. Power supply to the processing block 82 isobtained from the onboard battery 85 through the ignition switch 84.

Operation of the stepping motor type indicator of this embodiment isfurther described referring to FIGS. 5 and 7(a)-7(c). When the ignitionswitch 84 is off, the power is not supplied from the onboard battery 85so that no driving signals are fed into the exciting coils 21, 24(non-excited condition). In this embodiment, a stopping position P wherethe pointer 4 comes into contact with the stopper pin 7 and stays in thenon-excited condition is slightly offset from a zero point Z of thescale 30 on the dial plate 3 to a decreasing side (negative side) ofdial reading (FIG. 7(a)). When the ignition switch 84 is turned on andthe power is fed from the onboard battery 85 to the driving means 800,the driving means 800 supplies the driving signals to the exciting coils21, 24, whereby the pointer 4 is moved to the zero point Z (FIG. 7(b)).After the vehicle's engine has been started, the driving means 800controls the driving signals according to the input signal from theinput terminal 80 so that the pointer 4 gives an indicationcorresponding to the measurement value (FIG. 7(c)).

It is known that small magnetic forces (detent torques) are exerted onthe rotor 28 even in the non-excited condition when the power is off notonly in the stepping motor 2 used in the stepping motor type indicatorbut also in every PM-type stepping motor of this kind.

These magnetic forces (detent torques) are produced by magnetic circuitsformed between the multiple magnetic poles of the rotor 28 and theircorresponding pole teeth 232, 233, 262, 263 which have approximately thesame pole end area and are arranged and meshed together at specifiedregular intervals. The magnetic forces (detent torques) produce aplurality of stable points which attract and hold the rotor 28 atspecific positions. In most cases, the number of the stable points thusproduced is approximately equal to the number of the magnetic poles ofthe rotor 28 or to a multiple (or the reciprocal of a multiple) of thenumber of the magnetic poles of the rotor 28. Since the rotor 28 havinga total of 12 north and south poles are driven by a total of 24 poleteeth 232, 233, 262, 263 of both stators A, B in this embodiment, thereare formed 6, 12 or 24 stable points.

The existence of these stable points is important for the relationshipbetween the stepping motor 2 and the dial plate 3 especially in definingthe stopping position P of the pointer 4 in the stepping motor typeindicator which is constructed such that the pointer 4 is heldstationary by the stopper pin 7 in the non-excited condition. Therelationship between the stable points and the pointer 4 is defined asdescribed below in this embodiment.

FIG. 8 shows a case in which there are formed 12 stable points a-lgenerally equal intervals. (The supporting plates 29 are not illustratedin FIG. 8.) Among these stable points a-l, arbitrarily selected firstand second stable points a, b which are situated adjacent to each otheralong the increasing and decreasing directions of the pointer 4 areexplained here. At the stable point a situated on the decreasing side ofthe indication of the pointer 4, a magnetic force which tends to holdthe pointer 4 at the stable point a in a stable fashion is exerted onthe rotor 28. In the vicinity of the stable point a, rotational magneticforces MP1, MP2 which tend to attract the pointer 4 toward the stablepoint a are exerted on the rotor 28. Also at the stable point b situatedon the increasing side of the pointer 4, a magnetic force which tends tohold the pointer 4 in a stable fashion at the stable point b is exertedon the rotor 28. In the vicinity of the stable point b, rotationalmagnetic forces MP3, MP4 which tend to attract the pointer 4 toward thestable point b are exerted on the rotor 28.

The rotational magnetic forces MP2, MP3 exerted on the rotor 28 toattract the pointer 4 between the stable points a, b lessen toward amiddle point (unstable point) M between the stable points a, b andbecome most unstable at the middle point M. Thus, a stopping position Wof the rotor 28 determined by the stopper pin 7 is provided between thefirst and second stable points a, b situated adjacent to each otheralong the increasing and decreasing directions of the pointer 4 which isfixed to the rotor 28, and at a point offset from the middle point Mbetween the first and second stable points a, b toward the decreasingside of the indication of the pointer 4. In other words, the stoppingposition W of the rotor 28 is located at the point where the rotationalmagnetic force MP2, which tends to attract the pointer 4 toward thestable point a situated on the decreasing side of the indication of thepointer 4, is exerted on the rotor 28. With this arrangement, thepointer 4 situated at the stopping position P is biased toward thestopper pin 7 (or in a direction of contact with the stopper pin 7) bythe rotational magnetic force MP2 and can be held at the point of originP in a stable fashion. In this arrangement, it is desirable that thestopping position W of the rotor 28 be located neither at the middlepoint M nor at the stable point a, but it should be located at a pointwhere the rotational magnetic force MP2 directed to the stable point ais sufficiently exerted on the rotor 28.

In certain cases, however, such a stable point which would bias orattract the rotor 28 in a specific direction is not formedunambiguously, and if a stable point created is ambiguous and itsattracting force is too small, the stable point may not be able topositively hold the pointer 4 even when its stopping position P isdetermined in consideration of the stable point as described above.

Taking into account this potential problem, about half a terminalportion of one specific pole tooth 232 a chosen from the multiple poleteeth 232, 233, 262, 263 is cut away in this invention as shown in FIG.9. Accordingly, the pole end area of the pole tooth 232 a facing therotor 28 differs from the pole end areas of the other pole teeth 232,233, 262, 263, and this serves to make the individual stable points a-lunambiguous so that they produce strong attracting forces. Althoughabout half the terminal portion of one specific pole tooth 232 a is cutaway to differentiate its pole end area in the present embodiment, thepole end area of a particular pole tooth may be differentiated byforming the pole tooth in a smaller size from the beginning or byforming a hole in the pole tooth instead cutting it away.

In the earlier-described conventional construction in which theindividual pole teeth 232, 233, 262, 263 have the same pole end area, itis unlikely that large differences occur in the detent torques producedby the magnetic circuits formed between individual the magnetic poles ofthe rotor 28 and their corresponding pole teeth 232, 233, 262, 263 and,therefore, the detent torques are generally balanced around the curvedouter surface of the rotor 28 and the stable points tend to becomeambiguous and produce small holding forces. If the pole end area of thespecific pole tooth 232 a facing the rotor 28 is made different from thepole end areas of the other pole teeth 232, 233, 262, 263, there is madea distinct difference between the detent torque produced by the poletooth 232 a and the detent torques produced by the other pole teeth 232,233, 262, 263. Such variations in the detent torque create highlydistinguishable relative differences between the stable points a-l(which exert larger attracting forces on the rotor 28) and unstablepoints M (which exert smaller attracting forces on the rotor 28) whichare formed at specific intervals along the rotating direction of therotor 28. For this reason, the stable points a-l become more distinctand, as a consequence, they gain the ability to securely bias or attractthe rotor 28 toward its stopping position W to positively hold thepointer 4 at its stopping position P determined by the stopper pin 7.

In one aspect of the invention, the stepping motor type indicator ofthis embodiment so far described comprises the stepping motor 2 which isconstructed of the rotor 28 having the output shaft 27 and the first andsecond stators A, B having the yokes 23, 26, from which the multiplepole teeth 232, 233, 262, 263 extend toward the curved outer surface ofthe rotor 28, and the exciting coils 21, 24 for magnetizing the yokes23, 26, respectively, wherein the rotor 28 is driven to rotate inaccordance with the measurement value, the pointer 4 which is connectedto the output shaft 27 of the stepping motor 2 to indicate themeasurement value on the dial plate 3, and the stopper pin (rotorarresting means) 7 for stopping the pointer 4 at the specified positionon the dial plate 3. When the exciting coils 21, 24 are not excited,there are formed the multiple stable points a-l of the rotor 28 alongthe turning direction of the pointer 4 due to the magnetic forcesexerted between the rotor 28 and the yokes 23, 26. The stopping positionP of the rotor 28 (pointer 4) determined by the stopper pin 7 is set ata point closer to the stable point a which is located or the decreasingside of dial reading than the approximately middle point M between thearbitrarily selected two adjacent stable points a and b. Since the poleend area of the specific pole tooth 232 a chosen from the multiple poleteeth 232, 233, 262, 263 facing the rotor 28 is made different from thepole end areas of the other pole teeth 232, 233, 262, 263 in thisembodiment, the individual stable points a-l become unambiguous andproduce strong holding forces so that the attracting force exerted bythe stable point a on the pointer 4 (rotor 28) toward the stopper pin 7is increased, making it possible to positively suppress undesirablemovements of the pointer 4.

In another aspect of the invention, the stepping motor type indicator ofthis embodiment comprises the stepping motor 2 which is constructed ofthe rotor 28 having the output shaft 27 and the first and second statorsA, B having the yokes 23, 26, from which the multiple pole teeth 232,233, 262, 263 extend toward the curved outer surface of the rotor 28,and the exciting coils 21, 24 for magnetizing the yokes 23, 26,respectively, wherein the first and second stators A, B areconcentrically stacked and the rotor 28 is driven to rotate inaccordance with the measurement value, the driving means 800 whichsupplies the driving signals to the exciting coils 21, 24 of theindividual stators A, B to cause the rotor 28 to rotate in accordancewith the measurement value, the pointer 4 which is connected to theoutput shaft 27 to indicate the measurement value on the dial plate 3,and the stopper pin (rotor arresting means) 7 for stopping the pointer 4at the specified position on the dial plate 3. When the exciting coils21, 24 are not excited, there are formed the multiple stable points a-lof the rotor 28 along the turning direction of the pointer 4 due to themagnetic forces exerted between the rotor 28 and the yokes 23, 26. Thestopping position P of the rotor 28 (pointer 4) determined by thestopper pin 7 is set at a point closer to the stable point a which islocated on the decreasing side of dial reading than the approximatelymiddle point M between the arbitrarily selected two adjacent stablepoints a and b. As the pole end area of the specific pole tooth 232 achosen from the multiple pole teeth 232, 233, 262, 263 facing the rotor28 is made different from the pole end areas of the other pole teeth232, 233, 262, 263 in this embodiment, there is created a relativedifference between the pole end areas of the pole teeth 232 (232 a), 233of the first stator A facing the rotor 28 and those of the pole teeth262, 263 of the second stator B. Since the root-mean-square value of thedriving signal fed into the exciting coil 21 of the first stator A whosepole teeth 232 (232 a), 233 have smaller pole end areas facing the rotor28 is made higher than the root-mean-square value of the driving signalfed into the exciting coil 24 of the second stator B whose pole teeth262, 263 have larger pole end areas facing the rotor 28, the individualstable points a-l become unambiguous and produce strong holding forcesso that the attracting force exerted by the stable point a on thepointer 4 (rotor 28) toward the stopper pin 7 is increased, making itpossible to positively suppress undesirable movements of the pointer 4.Furthermore, it becomes possible to maintain good excitation balancebetween the individual stators A, B in a normal driven condition(excited condition) and thereby suppress adverse effects on rotationalcharacteristics of the rotor 28 during normal operation.

If the pole end area of the specific pole tooth 232 a chosen from themultiple pole teeth 232, 233, 262, 263 facing the rotor 28 is madedifferent from the pole end areas of the other pole teeth 232, 233, 262,263, the attracting force exerted on the pointer 4 (rotor 28) by thestable point a toward the stopper pin 7 is increased and undesirablemovements of the pointer 4 can be positively suppressed as describedabove. When the pole end area of the specific pole tooth 232 a facingthe rotor 28 is made different from the pole end areas of the other poleteeth 232, 233, 262, 263 and there is a relative difference between thepole end areas of the pole teeth 232 (232 a), 233 of the first stator Afacing the rotor 28 and those of the pole teeth 262, 263 of the secondstator B, a difference in excitation torque could occur between the twostators A, B in the normal operating condition (excited condition),affecting rotational characteristics of the rotor 28 in certain cases.Thus, the root-mean-square value of the driving signal fed into theexciting coil 21 of the first stator A whose pole teeth 232 (232 a), 233have smaller pole end areas facing the rotor 28 is made higher than theroot-mean-square value of the driving signal fed into the exciting coil24 of the second stator B whose pole teeth 262, 263 have larger pole endareas facing the rotor 28 in order to increase the excitation torqueproduced by the first stator A relative to the excitation torqueproduced by the second stator B and thereby achieve a proper balance ofexcitation torque between the two stators A, B. This makes it possibleto suppress such adverse effects on the rotational characteristics ofthe rotor 28 as an increase in the indication error of the pointer 4 anddegradation of smoothness of the movement of the pointer 4 when there isa difference in the pole end areas of the pole teeth 232, 233, 262, 263facing the rotor 28 between the two stators A, B.

Although the pole end area of the specific pole tooth 232 a is madesmaller in this embodiment, the pole end area of the specific pole tooth232 a may be made larger than the pole end areas of the other pole teeth232, 233, 262, 263.

Furthermore, although the pole end area of only one specific pole tooth232 a is differentiated in this embodiment, the pole end area of morethan one specific pole tooth 232 a may be made different from the poleend areas of the other pole teeth 232, 233, 262, 263.

An advantage gained by differentiating the pole end area of only onespecific pole tooth 232 a is that it is possible to reduce such adverseeffects on the rotational characteristics of the rotor 28 in the normaloperating condition (excited condition) as an increase in the indicationerror of the pointer 4 and degradation of smoothness of the movement ofthe pointer 4.

More specifically, although the rotational characteristics of the rotor28 in the normal operating condition (excited condition) would beadversely affected if the pole end areas of some of the pole teeth 232,233, 262, 263 are differentiated from the pole end areas of the otherpole teeth 232, 233, 262, 263, it is possible to minimize such adverseeffects on the rotational characteristics of the rotor 28 duringoperation in the excited condition when the pole end area of only onepole tooth 232 a is made different from the pole end areas of the otherpole teeth 232, 233, 262, 263.

In this embodiment, an angular position e0 taken by the pointer 4 whenthe driving means 800 outputs driving signals (see FIG. 6) correspondingto an electrical angle of 0° is matched with the stable point a producedin the non-excited condition as shown in FIG. 10(a).

This is because the driving means 800 outputs the driving signalscorresponding to the electrical angle of 0° when the ignition switch 84is turned on.

When the ignition switch 84 is off, the rotor 28 (pointer 4) isprohibited from rotating by the stopper pin 7 and remains stationary atthe stopping position P due to the attracting force exerted by thestable point a. When the ignition switch 84 is turned on, the drivingmeans 800 supplies the driving signals corresponding to the electricalangle of 0° into the exciting coils 21, 24. While a turning force whichcauses the pointer 4 to swing toward the angular position e0corresponding to the electrical angle of 0° is exerted on the rotor 28(pointer 4) at this point, the movement of the pointer 4 is stopped bythe stopper pin 7. At this time, the turning force which could turn thepointer 4 from its stopping position P determined by the stopper pin 7to the angular position e0 is exerted on the rotor 28 (pointer 4).

If, however, the stable point a and the angular position e0 taken by thepointer 4 when the driving means 800 outputs the driving signalscorresponding to the electrical angle of 0° are situated apart from eachother as shown in FIG. 10(b), an excessive torque (turning force)corresponding to the distance between the stable point a and the angularposition e0 is applied to the stopper pin 7. In this case, the pointer 4is likely to jump over the stopper pin 7 depending on the materials ofthe pointer 4 and the stopper pin 7 themselves. In this circumstance, ifthe stable point a and the angular position e0 taken by the pointer 4when the driving means 800 outputs the driving signals corresponding tothe electrical angle of 0° are matched with each other as shown in FIG.10(a), the torque (turning force) applied to the stopper pin 7 decreasesas a result of the reduction in the distance between the stable point aand the angular position e0, making it possible to prevent the jumpingof the pointer 4. Although the stable point a and the angular positione0 taken by the pointer 4 when the driving means 800 outputs the drivingsignals corresponding to the electrical angle of 0° are matched witheach other in this embodiment, they need not necessarily coincideexactly with each other but should just be situated as close to eachother as would be sufficient to prevent the jumping of the pointer 4.

In a case where the driving means 800 for driving the stepping motor 2supplies the driving signals whose output waveforms have a phasedifference of about 90° in terms of electrical angle, it is possible toprevent the pointer 4 from moving from its stopping position Pdetermined by the stopper pin 7 when the driving means 800 outputs thedriving signals corresponding to the electrical angle of 0° (when theignition switch 84 is turned on) as described above, if the angularposition e0 taken by the pointer 4 when the driving means 800 outputsthe driving signals corresponding to the electrical angle of 0° and thestable point a produced in the non-excited condition are matched exactlyor approximately with each other.

As described above in detail, a stepping motor type indicator in oneaspect of the invention comprises a stepping motor which is constructedof a rotor formed of permanent magnets and having an output shaft, and astator having a yoke, from which a plurality of pole teeth extend towarda curved outer surface of the rotor, and an exciting coil formagnetizing the yoke, in which the rotor is driven to rotate inaccordance with a measurement value, a pointer which is connected to theoutput shaft of said stepping motor and indicates the measurement valueon a dial plate, and a rotor stopper for stopping said pointer at aspecified stopping position on the dial plate, wherein the stoppingposition of said pointer determined by said rotor stopper is set at apoint closer to a stable point which is located on a decreasing side ofdial reading of said pointer than an approximately middle point betweentwo adjacent stable points arbitrarily selected from a plurality ofstable points of the rotor which are formed along the turning directionof said pointer due to magnetic forces exerted between the rotor and theyoke in a non-excited condition of the exciting coil, and the pole endarea of a specific pole tooth chosen from said multiple pole teethfacing the rotor is made different from the pole end areas of the otherpole teeth facing the rotor. According to this construction, it ispossible to provide a stepping motor type indicator in which individualstable points produced in the non-excited condition are made unambiguousand undesirable movements of the pointer can be suppressed.

In another aspect of the invention, a stepping motor type indicatorcomprises a stepping motor which is constructed of a rotor formed ofpermanent magnets and having an output shaft, and first and secondstators individually having yokes, from which a plurality of pole teethextend toward a curved outer surface of the rotor, and exciting coilsfor magnetizing the yokes, in which said first and second stators areconcentrically stacked and said rotor is driven to rotate in accordancewith a measurement value, a driver which supplies driving signals to theexciting coils of said individual stators to cause the rotor to rotatein accordance with the measurement value, a pointer which is connectedto the output shaft of said stepping motor and indicates the measurementvalue on a dial plate, and a rotor stopper for stopping said pointer ata specified stopping position on the dial plate, wherein the stoppingposition of said pointer determined by said rotor stopper is set at apoint closer to a stable point which is located on a decreasing side ofdial reading of said pointer than an approximately middle point betweentwo adjacent stable points arbitrarily selected from a plurality ofstable points of the rotor which are formed along the turning directionof said pointer due to magnetic forces exerted between the rotor and theyokes in a non-excited condition of the exciting coils, the pole endarea of a specific pole tooth chosen from said multiple pole teethfacing the rotor is made different from the pole end areas of the otherpole teeth facing the rotor so that a relative difference is createdbetween the pole end areas of the pole teeth of said first stator facingthe rotor and those of the pole teeth of said second stator, and theroot-mean-square value of the driving signal fed into the exciting coilof said first stator whose pole teeth have smaller pole end areas facingthe rotor is made higher than the root-mean-square value of the drivingsignal fed into the exciting coil of said second stator whose pole teethhave larger pole end areas facing the rotor. According to thisconstruction, it is possible to provide a stepping motor type indicatorin which individual stable points produced in the non-excited conditionare made unambiguous and undesirable movements of the pointer can besuppressed. Furthermore, it becomes possible to maintain good excitationbalance between the two stators in a normal driven condition (excitedcondition) and thereby suppress adverse effects on rotationalcharacteristics of the rotor during normal operation.

In one specific form of the invention, the pole end area of only onespecific pole tooth is made different from the pole end areas of theother pole teeth. This makes it possible to reduce adverse effects onthe rotational characteristics of the rotor in the normal operatingcondition (excited condition).

In a further aspect of the invention, a stepping motor type indicatorcomprises a stepping motor which is constructed of a rotor formed ofpermanent magnets and having an output shaft, and first and secondstators individually having yokes, from which a plurality of pole teethextend toward a curved outer surface of the rotor, and exciting coilsfor magnetizing the yokes, in which said first and second stators areconcentrically stacked and said rotor is driven to rotate in accordancewith a measurement value, a driver which supplies driving signals whoseoutput waveforms have a phase difference of about 90° in terms ofelectrical angle to the exciting coils of said individual stators tocause the rotor to rotate in accordance with the measurement value, apointer which is connected to the output shaft of said stepping motorand indicates the measurement value on a dial plate, and a rotor stopperfor stopping said pointer at a specified stopping position on the dialplate, wherein the stopping position of said pointer determined by saidrotor stopper is set at a point closer to a stable point which islocated on a decreasing side of dial reading of said pointer than anapproximately middle point between two adjacent stable pointsarbitrarily selected from a plurality of stable points of the rotorwhich are formed along the turning direction of said pointer due tomagnetic forces exerted between the rotor and the yokes in a non-excitedcondition of the exciting coils, the pole end area of a specific poletooth chosen from said multiple pole teeth facing the rotor is madedifferent from the pole end areas of the other pole teeth facing therotor, and an angular position taken by the said pointer when saiddriver outputs the driving signals corresponding to an electrical angleof 0° and said stable point in the non-excited condition are matchedexactly or approximately with each other. According to thisconstruction, it is possible to provide a stepping motor type indicatorin which individual stable points produced in the non-excited conditionare made unambiguous and undesirable movements of the pointer can besuppressed. Furthermore, it becomes possible to prevent the pointer fromdeviating from its stopping position when the stepping motor typeindicator is powered on.

What is claimed:
 1. A stepping motor type indicator comprising astepping motor which is constructed of a rotor formed of permanentmagnets and having an output shaft, and first and second statorsindividually having yokes, from which a plurality of pole teeth extendtoward a curved outer surface of the rotor, and exciting coils formagnetizing the yokes, in which said first and second stators areconcentrically stacked and said rotor is driven to rotate in accordancewith a measurement value; a driver which supplies driving signals to theexciting coils of said individual stators to cause the rotor to rotatein accordance with the measurement value; a pointer which is connectedto the output shaft of said stepping motor and indicates the measurementvalue on a dial plate; and a rotor stopper for stopping said pointer ata specified stopping position on the dial plate, wherein the stoppingposition of said pointer determined by said rotor stopper is set at apoint closer to a stable point which is located on a decreasing side ofdial reading of said pointer than an approximately middle point betweentwo adjacent stable points arbitrarily selected from a plurality ofstable points of the rotor which are formed along the turning directionof said pointer due to magnetic forces exerted between the rotor and theyokes in a non-excited condition of the exciting coils, the pole endarea of a specific pole tooth chosen from said multiple pole teethfacing the rotor is made different from the pole end areas of the otherpole teeth facing the rotor so that a relative difference is createdbetween the pole end areas of the pole teeth of said first stator facingthe rotor and those of the pole teeth of said second stator, and theroot-mean-square value of the driving signal fed into the exciting coilof said first stator whose pole teeth have smaller pole end areas facingthe rotor is made higher than the root-mean-square value of the drivingsignal fed into the exciting coil of said second stator whole pole teethhave larger pole end areas facing the rotor.
 2. A stepping motor typeindicator according to claim 1, wherein the pole end area of only onespecific pole tooth is made different from the pole end areas of theother pole teeth.
 3. A stepping motor type indicator according to claim2, wherein an angular position taken by said pointer when said driveroutputs the driving signals corresponding to an electrical angle of 0°and said stable point in the non-excited condition are matched exactlyor approximately with each other.
 4. A stepping motor type indicatoraccording to claim 1, wherein an angular position taken by said pointerwhen said driver outputs the driving signals corresponding to anelectrical angle of 0° and said stable point in the non-excitedcondition are matched exactly or approximately with each other.
 5. Astepping motor type indicator comprising a stepping motor which isconstructed of a rotor formed of permanent magnets and having an outputshaft, and first and second stators individually having yokes, fromwhich a plurality of pole teeth extend toward a curved outer surface ofthe rotor, and exciting coils for magnetizing the yokes, in which saidfirst and second stators are concentrically stacked and said rotor isdriven to rotate in accordance with a measurement value; a driver whichsupplies driving signals whose output waveforms have a phase differenceto the exciting coils of said individual stators to cause the rotor torotate in accordance with the measurement value; a pointer which isconnected to the output shaft of said stepping motor and indicates themeasurement value on a dial plate; and a rotor stopper for stopping saidpointer at a specified stopping position on the dial plate, wherein thestopping position of said pointer determined by said rotor stopper isset at a point closer to a stable point which is located on a decreasingside of dial reading of said pointer than an approximately middle pointbetween two adjacent stable points arbitrarily selected from a pluralityof stable points of the rotor which are formed along the turningdirection of said pointer due to magnetic forces exerted between therotor and the yokes in a non-excited condition of the exciting coils,the pole end area of a specific pole tooth chosen from said multiplepole teeth facing the rotor is made different from the pole end areas ofthe other pole teeth facing the rotor to increase an attractive force atthe stable points, and an angular position taken by the said pointerwhen said driver outputs the driving signals corresponding to anelectrical angle of 0° and said stable point in the non-excitedcondition are matched exactly or approximately with each other.